Test 27 - Earth Dam Seepage, Stability and Failure Modes

Question 1 ప్రశ్న 1

Why must seepage be controlled in an earth dam even though some seepage through soil is normally unavoidable?

A Because any measurable seepage proves that the dam has already failed B Because uncontrolled seepage can raise pore pressures, destabilize the downstream slope, and initiate internal erosion or piping C Because seepage affects only reservoir evaporation and has no structural significance D Because seepage control is required only to increase the unit weight of the embankment

Question 2 ప్రశ్న 2

In an earth-dam cross-section under steady seepage, what is the phreatic surface?

A The line on which total stress is zero B The boundary below which the soil has zero unit weight C The seepage surface along which pore-water pressure is atmospheric, represented as a line in a two-dimensional cross-section D The line joining all points of maximum shear strength

Question 3 ప్రశ్న 3

Which statement about pore-water pressure below the phreatic surface is generally correct for steady seepage in an earth dam?

A Positive pore-water pressure may exist below the phreatic surface and reduces effective stress relative to the same total stress with lower pore pressure B Pore-water pressure must be zero everywhere below the phreatic surface C Only negative pore-water pressure can exist below the phreatic surface D Pore-water pressure affects discharge but cannot influence slope stability

Question 4 ప్రశ్న 4

What is the principal concern if the phreatic surface in an earth dam rises unusually high and approaches or intersects an unprotected downstream slope?

A It necessarily improves downstream slope stability by increasing effective stress B It guarantees that no seepage can leave the dam C It eliminates the possibility of internal erosion D It can saturate the downstream zone, increase pore pressures, and promote seepage emergence, sloughing, or instability

Question 5 ప్రశ్న 5

Why is a chimney filter or drain connected to a horizontal drainage blanket commonly placed downstream of the impervious core?

A To force the phreatic surface to emerge higher on the downstream slope B To intercept seepage, protect against migration of fine particles, and convey water safely toward the downstream toe C To make the downstream shell completely impermeable D To replace the need for a low-permeability core

Question 6 ప్రశ్న 6

Which description best represents backward-erosion piping in the foundation or embankment of an earth dam?

A Uniform compression of the dam under self-weight B Surface erosion caused only by rainfall on the downstream face C Progressive removal of soil beginning near a seepage exit and advancing backward toward the water source, potentially forming a continuous pipe D Settlement of the crest without any movement of soil by water

Question 7 ప్రశ్న 7

A transverse crack develops through the impervious core because of differential settlement, and reservoir water begins flowing through the crack and eroding its walls. Which failure mechanism is most directly represented?

A Concentrated-leak internal erosion B Pure bearing-capacity failure with no seepage involvement C Wave erosion of the upstream riprap D Only surface sloughing caused by rainfall

Question 8 ప్రశ్న 8

During inspection of an earth dam, which observation most strongly suggests that internal erosion may be actively transporting soil?

A A stable, unchanged clear seepage flow at a designed drain outlet B Dry grass on the upper downstream slope C A small amount of evaporation from the reservoir D A new or increasing muddy seepage discharge carrying visible soil particles

Question 9 ప్రశ్న 9

What combination of properties is required of a granular filter protecting a fine-grained earth-dam core?

A It should be so fine that essentially no seepage can pass through it B It should be fine enough to retain the protected soil yet sufficiently permeable to pass seepage without excessive pressure buildup C It should intentionally permit the core particles to migrate into the drain D It should always have exactly the same gradation as the core

Question 10 ప్రశ్న 10

Which statement correctly distinguishes the functions of a filter and a drain in an earth dam?

A A filter blocks all water while a drain blocks all soil and water B A filter is used only on the crest, whereas a drain is used only on the upstream slope C A filter primarily protects soil against particle migration while allowing water passage, whereas a drain must also provide adequate capacity to collect and convey seepage D There is no functional distinction because every filter must discharge directly into the reservoir

Question 11 ప్రశ్న 11

What is the principal benefit of a properly designed toe drain at the downstream toe of an earth dam?

A It provides a controlled outlet for intercepted seepage and helps prevent uncontrolled saturation and seepage emergence on the downstream slope B It raises the downstream phreatic surface to the crest C It acts as the primary reservoir water barrier D It increases the hydraulic gradient at the downstream exit deliberately

Question 12 ప్రశ్న 12

What is the main seepage-control effect of a cutoff trench or cutoff wall extending into a pervious foundation beneath an earth dam?

A It deliberately shortens the seepage path and increases exit gradient B It causes all seepage to emerge on the downstream face C It makes internal drainage unnecessary under every foundation condition D It reduces underseepage by increasing flow resistance and, depending on its depth and continuity, can reduce downstream gradients and uplift

Question 13 ప్రశ్న 13

In an idealized seepage calculation, hydraulic conductivity is 2 × 10^-6 m/s, hydraulic gradient is 0.50, and the flow area normal to seepage is 30 m² per metre length of dam. Using Darcy's law, what is the seepage discharge per metre length?

A 0.003 L/s B 0.030 L/s C 0.30 L/s D 3.0 L/s

Question 14 ప్రశ్న 14

A seepage path has a total head loss of 12 m over an idealized flow length of 60 m. What is the average hydraulic gradient along that path?

A 0.05 B 0.12 C 0.20 D 5.00

Question 15 ప్రశ్న 15

For an ideal saturated cohesionless soil, the critical upward hydraulic gradient may be estimated as i_cr = (G_s - 1)/(1 + e). If G_s = 2.65 and void ratio e = 0.65, what is i_cr?

A 1.00 B 0.50 C 1.65 D 2.65

Question 16 ప్రశ్న 16

At a point in a saturated earth-dam slope, the total normal stress is 180 kPa and pore-water pressure is 70 kPa. What is the effective normal stress at that point?

A 70 kPa B 180 kPa C 250 kPa D 110 kPa

Question 17 ప్రశ్న 17

Under prolonged full-reservoir conditions after steady seepage has developed, which slope of a conventional earth dam commonly requires particular seepage-related stability attention?

A The upstream slope only, because the downstream slope remains completely dry B The downstream slope, because seepage pore pressures and the phreatic surface can reduce effective stress there C Neither slope, because steady seepage eliminates pore pressure D Only the crest, because slope stability is independent of seepage

Question 18 ప్రశ్న 18

Why is rapid drawdown of the reservoir commonly a critical loading condition for the upstream slope of an earth dam?

A Because reservoir lowering instantaneously increases external water support on the upstream face B Because rapid drawdown always removes pore pressure from low-permeability fill before the water level falls C Because external reservoir support can fall rapidly while pore-water pressures within low-permeability upstream fill dissipate much more slowly D Because it makes the downstream slope weightless

Question 19 ప్రశ్న 19

For a simplified slope-stability calculation, the available resisting shear force along an assumed slip surface is 1950 kN per metre length and the mobilized driving shear force is 1300 kN per metre length. What is the corresponding factor of safety?

A 1.50 B 0.67 C 2.50 D 650

Question 20 ప్రశ్న 20

What is meant by sloughing of the downstream slope of an earth dam in a seepage-distress context?

A Intentional excavation of the slope during construction B Wave erosion confined to the upstream face C A purely elastic deformation that cannot involve soil movement D Local or shallow sliding and softening of downstream slope material, often associated with saturation or seepage emerging from the embankment

Question 21 ప్రశ్న 21

Why is uncontrolled overtopping especially dangerous for a conventional earth dam?

A Because overtopping immediately eliminates the reservoir hydrostatic force B Because flow over the crest and downstream face can rapidly erode earth materials, enlarge a breach, and lead to progressive failure C Because earthfill becomes completely impermeable once water crosses the crest D Because overtopping affects only the upstream riprap and not the embankment

Question 22 ప్రశ్న 22

How can differential settlement increase the risk of internal erosion in an earth dam?

A It always closes existing cracks and makes the core more impermeable B It has no relationship to seepage because settlement is only vertical C It can produce cracks or poorly compressed interfaces that become preferential seepage paths and allow concentrated-leak erosion to initiate D It prevents reservoir water from contacting the core

Question 23 ప్రశ్న 23

Why is a transverse crack through an earth-dam core generally of greater internal-erosion concern than a longitudinal crack that does not cross the core?

A A transverse crack can provide a direct preferential seepage path from the reservoir side toward the downstream side B Longitudinal cracks always transmit more reservoir flow than transverse cracks C Transverse cracks cannot be protected by filters D Longitudinal cracks never affect slope stability

Question 24 ప్రశ్న 24

Which monitoring pattern should cause particular concern for seepage and stability performance of an earth dam?

A Unchanged piezometer readings and stable clear drain flow over many comparable reservoir cycles B A gradual reduction in reservoir evaporation during cool weather C Routine vegetation growth with no change in seepage or deformation D Rising piezometric levels, increasing or newly located seepage, turbidity, wet spots, sinkholes, cracking, or unexplained settlement

Question 25 ప్రశ్న 25

An earth dam develops a new wet area on its downstream slope, drain discharge is increasing and becoming muddy, and nearby piezometers show rising pore-water levels. What is the most appropriate engineering interpretation?

A The observations prove that the dam is improving because more water is leaving the reservoir B The combination is potentially indicative of deteriorating seepage control or internal erosion and requires prompt investigation while maintaining or restoring safe filtered drainage rather than simply blocking the seepage exit C The muddy flow should be plugged immediately at the downstream surface without considering pore-pressure consequences D No action or evaluation is needed unless the crest has already overtopped

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